A real long shot here and likely not an issue, but one that I experienced on an auto engine some 15 years ago.
Old hoses get weak. Especially on the suction side of the water pump if you engine has such. Once warmed up, they can collapse, cutting off the flow to some degree, especially ones that are bent if you have any of those feeding the water pump.
I think it was an old Mustang, and the lower hose had a bend in it and an internal spring to prevent collapse. If you removed the spring or had an old hose without the supporting spring, it would collapse with warm up and restrict flow causing overheating.
Like I said a real long shot, but maybe check all your hoses for internal weak spots, and also for internal swelling. Age and heat can cause the internal rubber to swell.
I've seen this hose collapse thing also. Usually someone has not installed a spring in a long hose or the hose has a sharp bend and flattens when it gets hot.
A head gasket leaking will cause overheating. (also a cracked head usually at the vicinity of a exhaust valve) Sometimes you can see bubbles in the top of the radiator when the engine is first started cold. Take the radiator cap off when the engine is cold, let the engine idle and watch the top of a full radiator for any bubbles, even slight bubbles. DO NOT TRY THIS WITH A HOT ENGINE. Sometimes you can compare the color of all the spark plugs and suspect the leakers to be darker color and sometimes you can actually see anti-freeze drops on the plug before the engine gets hot.
On some engines that do not have a thermostat bypass hose or bypass and exhaust gases are getting into the coolant in the block you can leave the thermostat out and the engine will run cooler, sometimes for a long time cooler, BECAUSE if the engine don't have a thermostat bypass and exhaust gas is getting into the block and gets trapped under the thermostat's pill holding back the coolant and it goes into a runaway really fast when the leaking gasket gets really leaking bad.
Sometimes after all else is eliminated you just have to break down and pull the head, inspect both the head and the gasket and install a new head gasket. If you do not see a cracked head get the head magnafluxed. (and don't hurt the messenger)I've been their on these type things and they are not fun. Main thing is too stay safe and not get scalded with hot water/antifreeze.
I've never seen where you tried BACKFLUSHING the block with the thermostat removed. If you do such, watch for any nasty orange rust coming out.
I would back flush the block before removing the head to check the gasket, backflushing a nasty block keeps the trash from being circulated into your clean radiator.
Very doubtful that your water pump is bad.
You said it was ok when you parked it before winter. (did it have good strong anti-freeze installed???)